The commercial manufacture of hydrofluoric acid (“HF”) typically consists of heating a mixture of a flouride bearing material, such as fluorspar and an acid feed stream, which is usually rich in acid, such a sulfuric acid, to produce a gaseous reaction product rich in HF. Such processes are disclosed in each of the following U.S. Patents, each of which is incorporated herein by reference: U.S. Pat. Nos. 4,292,289 and 3,167,391. The acid feed stream in many cases comprises a mixture of acids, with the acid mixture frequently comprising a major proportion of sulfuric acid and minor proportions hydrofluoric acid and fluorosulfonic acid. The acid feed stream also typically contains water. The efficiency and effectiveness of such HF manufacturing processes is a function of the relative amount of water in the acid feed stream.
At least a portion of the water contained in an acid mixture is produced via an equilibrium reaction, such as that involved in the three acid mixture shown in Formula 1 below.H2SO4+HF⇄HSO3F+H2O  (1)The water produced by such a reaction is said to be “bound” to the reaction and generally must remain in the acid mixture to ensure a higher-quality product. As “bound water” is removed from the above noted acid mixture, the reaction will be driven toward the production of fluorosulfonic acid, and as a result the HF produced in the manufacturing process will tend to contain undesirably higher amounts of fluorosulfonic acid and other unwanted contaminants. Thus, the removal of “bound” water from the acid feed stream generally results in a lower quality HF product.
Additionally, the aforementioned acid mixture generally contains some amount of “free water,” that is, any water present in an amount greater than the equilibrium concentration. Free water is introduced into the acid feed stream in a variety of ways, including: contaminated acid materials, atmospheric leaks, recycled process streams, in-line breaks for maintenance, and the like. Such free water is generally undesirable. For example, the presence of free water in the acid feed results in a process reaction which is more erosive and corrosive to the reaction vessel, furnace and other equipment used in the manufacture of HF in comparison to reactions in which the acid feed contains no free water.
Unfortunately, due at least in part to the highly corrosive nature of the aforementioned acid mixtures, traditional methods for regulating the water content in the acid feed stream tend to be relatively inaccurate and time-consuming. For example, one traditional method for controlling the amount of water in an acid mixture comprises the step of calculating water concentration as disclosed in “Procedure for the Determination of the Composition of HF/H2SO4/FSO3H/H2O Solutions” La Chimica e l'Industria, 62/3 (1980) pp. 189-192, L. Marangoni and R. Madiolo, incorporated herein by reference. This calculation method requires removing a sample from the acid feed in an HF production process and subjecting the sample to two titration processes and a distillation process. Based on these processes steps, an estimate of the amount of free water is made based on calculated concentrations of the water and acid components in the mixture. A system operator then considers these estimates in his/or her efforts to optimize the process.
However, as indicated in the Marangoni and Madiolo article, the concentrations used to control water content via the aforementioned method are inexact values which reflect the imprecisions of each of the three analytical methods. Accordingly, the prior art method tends to be inaccurate and imprecise in the control of water concentration. Additionally, the aforementioned method requires significant time to remove and analyze a sample as described. Thus, any control action that is taken may be ineffective, or in fact detrimental, because of the undesirably large time interval between sampling and actual implementation of process modification.
Recognizing these and other drawbacks of the prior art, the present inventors have perceived a need for a new, efficient and more accurate method for controlling the water concentration of an acid feed stream. These and other objects are achieved by the present invention as described below.